土工合成材料在软土地基处理中的应用

本文主要介绍软土地基的加固处理，利用土工合成材料加筋土垫层联合碎石垫层综合解决软土地基的承载力不足及冻胀问题。对于软土地基二维固结变形，瞬时沉降在地基总沉降量中占有很大的比例，而充分利用表面硬壳层及均匀慢速加载均对减少地基沉降有着显著的效果。     

桩-网复合地基承载及变形特性的有限元分析

采用ABAQUS有限元软件,通过有限元数值模拟的方法分析了路堤荷载作用下桩-网复合地基中土工合成材料刚度、垫层厚度、桩体模量以及桩间距对复合地基的荷载传递特性、桩-土应力比、路基的表面沉降及侧向位移的影响。总结、分析计算结果,获得了桩-网复合地基承载及变形的一些基本特性,如：增加土工合成材料刚度,可显著地减小桩-土差异沉降和路基侧向位移,并增加桩-土应力比;增加垫层厚度,可明地改善桩-土荷载分担比和桩-土应力比等力学性状。这些结果对桩-网复合地基的设计与施工具有一定的指导意义。     

桩承土工合成材料加筋垫层复合地基作用原理及应用

桩承土工合成材料加筋垫层法是近些年发展起来的一种有效的软土地基加固方法。简要地介绍了桩承土工合成材料加筋垫层法的设计和使用情况,主要包括其作用机理、设计方法和工程应用。桩承土工合成材料加筋垫层法主要适用于软土地基下有承载力较大的坚硬土层或岩层,上部有一定的填土高度,工期紧张,对总沉降和不均匀沉降有要求比较严格的情况。目前该方法已经被广泛应用于公路、铁路、桥台后过渡段、分段挡土墙、旧路加宽和房屋建筑的一些实际工程中。     

浙江台州高速公路一期工程软土地基综合处理效果的分析――以浙江台州黄岩软土地基试验路段为例

浙江台州高速公路为主要由淤泥、淤泥质粘土、淤泥质亚粘土组成的软土地基,经过预压法、轻质路堤法、土工合成材料加筋垫层等方法的综合处理,效果明显。在填筑过程中,使路堤保持稳定;在预压阶段,加速了地基的沉降,缩短了预压期;在路堤与桥头衔接部位,减少了两者之间的差异沉降;在高速公路建设的全过程中,增强了路堤的整体性能,提高了软土地基的力学强度,保证了全路段投入运营后的稳定和安全。      

CFG桩复合地基褥垫层效用的有限元分析

复合地基上褥垫层的设计是桩体复合地基设计的关键技术,通过有限元数值模拟分析,得到了桩土沉降与荷载之间的关系,获得了褥垫层厚度及其压缩模量对桩土应力比、桩土沉降等的影响规律。该研究为复合地基褥垫层厚度的设置及选择和配制合理的垫层材料提供了依据,有利于复合地基的优化设计。     

垫层对水泥土搅拌桩复合地基沉降的影响研究

根据静载荷试验和数值模拟的结果,研究垫层厚度对水泥土搅拌桩复合地基沉降的影响规律。针对珠海地区广泛存在的淤泥质土,进行了天然地基和4种垫层厚度的水泥土搅拌桩复合地基现场静载荷试验。在地基中预埋沉降管和沉降磁环,利用带刻度的探头感应加载过程中沉降磁环位置的变化,获取地基的分层沉降量,据此分析复合地基的沉降规律。在现场试验的基础上,对水泥土搅拌桩4桩复合地基进行了数值分析,将数值计算得到的不同垫层厚度复合地基中桩、土的分层沉降进行比较,结果表明垫层的设置减小了桩的沉降量,但过厚的垫层不能够有效地提高复合地基的承载力,反而会增大沉降。建议对褥垫层厚度的选取,除了要综合承载力和沉降两方面因素之外,还要考虑实际的地质条件。垫层厚度可取20～40 cm,当淤泥层位于桩身中部以下取高值,位于桩身中上部取低值。     

路堤下现浇薄壁管桩复合地基工作特性分析

现浇薄壁管桩和土工格栅加筋碎石垫层所组成的复合地基加固软弱路基具有经济合理，施工时间短，承载力高，沉降减小明显等优点。采用非线性有限元对路堤荷载作用下的现浇薄壁管桩复合地基的工作特性进行了分析，结果表明：在路堤荷载作用下，路堤填土材料中的拱效应、土工格栅的拉膜效应或加筋垫层的刚性垫层效应、桩土间刚度差异引起的应力集中效应保证了大部分荷载由桩承担，有效地减小了总沉降和不均匀沉降。同时对荷载传递机理的影响因素做了分析.     

CFG桩复合地基褥垫层三维弹塑性分析

在CFG桩复合地基中, 褥垫层的设置厚度和模量对于复合地基的承载及变形特性有较大的影响。本文建立了CFG桩复合地基的三维弹塑性数值分析模型,研究了当采用不同厚度和模量的褥垫层时,复合地基沉降、桩端沉降、褥垫层压缩量和桩土应力等参数的变化规律。结果表明复合地基沉降和褥垫层压缩量随着垫层厚度的增加而增大,而随着垫层模量的增大而减小;垫层厚度和模量的变化对于桩端沉降的影响较小,从工程角度可认为桩端沉降不随垫层厚度与垫层模量的变化而变化;桩土应力比随垫层厚度的增大而减小,随垫层模量的增大而增大。本文建立的分析模型还可进一步对CFG桩复合地基的其他参数进行相应的分析计算,分析结果可以为复合地基的设计和施工提供相应的理论依据。     

沉管隧道含垄沟卵石垫层变形特性试验研究

采用含垄沟卵石垫层作为大型沉管隧道的基础是一种创新,为研究水下该垫层在工程荷载下的变形特性,开展了7种不同工况模型静载试验,得到各工况下卵石垫层沉降量、压缩模量等参数以及荷载值与沉降量之间的关系,分析了垫层厚度、垄沟尺寸、预压荷载、级配大小等指标对卵石垫层力学特性的影响。试验结果表明,不同于碎石垫层卵石垫层承受的竖向荷载与其沉降量开始成非线性变化;随着荷载值的增加,垫层沉降速率逐渐减小,垫层被压密一定程度后荷载与沉降趋于线性关系;卵石的最大粒径越大,垫层的压缩模量越大,沉降量越小;预压导致卵石垫层再压缩时沉降明显减小,且压缩模量增大;垄沟的间距越大,卵石垫层的压缩模量越小,沉降量越大。垫层厚度越大,卵石垫层的压缩模量与沉降越大。研究成果可为沉管隧道垫层的设计提供参考。     

新近吹填淤泥地基负压传递特性及分布模式研究

为深入了解无砂垫层真空预压技术加固新近吹填淤泥地基过程中的负压传递特性和分布模式,依托珠海港高栏港区某新近吹填淤泥地基处理工程开展了现场试验研究,研究结果表明：（1）土工合成材料水平排水垫层中的负压损失程度较严重,至少为16%;整个排水系统内的负压损失程度也较严重,最严重的高达57%。（2）不同深度处土体中的负压传递特性一定程度上受相应位置处淤泥自重沉积规律的影响,粗颗粒含量高的深度位置,其负压衰减程度相应较低;但总体来讲,负压从竖井向土体传递过程中的损失程度很严重,至少67%,这是新近吹填淤泥地基经无砂垫层真空预压技术加固后土体强度增长有限的主要原因。最后,明确给出了该类地基的负压分布模式：水平排水垫层中的负压可考虑为随时间变化的线性衰减模式,而竖向排水体中的负压可考虑为随时间变化的非线性衰减模式。     

Improvement of soft clay using installation of geosynthetic-encased stone columns: numerical study

The use of geosynthetic-encased stone columns as a method for soft soil treatment is extensively used to increase the bearing capacity and reduce the settlement of raft foundations and the foundation of structures like embankments. Pre-strain is an effect occurring in the encasement during stone column installation due to the compaction of the stone material. The present study uses the finite element program Plaxis to perform a numerical analysis of the soft clay bed reinforced by geosynthetic-encased stone columns. An idealization is proposed for simulation of installation of geosynthetic-encased stone columns in soft clay based on the unit-cell concept. In the analyses, initially, the validity of the analysis of the single column-reinforced soil in the unit-cell model was performed through comparison with the group columns. Then, by considering a unit-cell model, the finite element analyses were carried out to evaluate the stiffness of the reinforced ground to estimate the settlement. The results of the analyses show that the improved stiffness of the encased stone column is not only due to the confining pressure offered by the geosynthetic after loading, but the initial strain of the geosynthetic that occurred during installation also contributes to the enhancement of the stiffness of the stone column and the reduction of the settlement.     

Response of multilayer geosynthetic-reinforced bed resting on soft soil with stone columns

In the present study, a mechanical model has been developed to study the behavior of multilayer geosynthetic-reinforced granular fill over stone column-reinforced soft soil. The granular fill and geosynthetic reinforcement layers have been idealized by Pasternak shear layer and rough elastic membranes, respectively. The Kelvin–Voight model has been used to represent the time-dependent behavior of saturated soft soil. The stone columns are idealized by stiffer springs and assumed to be linearly elastic. The nonlinear behavior of the soft soil and granular fill is considered. The effect of consolidation of soft soil due to inclusion of the stone columns on settlement response has also been included in the model. Plane strain conditions are considered for the loading and reinforced foundation soil system. An iterative finite difference scheme is applied for obtaining the solution and results are presented in nondimensional form. It has been observed that if the soft soil is improved with stone columns, the multilayer reinforcement system is less effective as compared to single layer reinforcement to reduce the total settlement as there is considerable reduction in the total settlement due to stone column itself. Multilayer reinforcement system is effective for reducing the total settlement when stone columns are not used. However, multilayer reinforcement system is effective to transfer the stress from soil to stone column. The differential settlement is also slightly reduced due to application of multiple geosynthetic layers as compared to the single layer reinforcement system.     

A State-of-the-Art Review of Stone/Sand-Column Reinforced Clay Systems

This paper presents a state-of-the-art review of published research papers and reports that focus on the modeling, testing, and analysis of soft clays that are reinforced with sand/stone columns in relation to bearing capacity and settlement considerations. The review is presented in chronological order to shed light on the development of this field of research in the last 40+ years. The objective of the study is to assemble published results from field, laboratory, and numerical investigations of sand/stone columns in clay in one resource to provide future researchers and designers with easy access to information and data. The majority of the reviewed papers include an experimental component that is based on field or laboratory scale tests (1-g, triaxial, or centrifuge) conducted on clay specimens reinforced with partially or fully penetrating, encased or ordinary, stone or sand columns that were installed as single columns or as column groups. Some papers included numerical experiments that were based on finite element models, while others presented analytical solutions for modeling the response of the composite system. A compilation of the important findings from physical, numerical, and analytical models in addition to a summary table that facilitates access to information from various research efforts are presented in the paper.     

Axi-symmetric Analysis of Geosynthetic-reinforced Granular Fill-soft Soil System with Group of Stone Columns

The paper presents a mechanical model to predict the behavior of geosynthetic-reinforced granular fill resting over soft soil improved with group of stone columns subjected to circular or axi-symmetric loading. The saturated soft soil has been idealized by spring-dashpot system. Pasternak shear layer and rough elastic membrane represent the granular fill and geosynthetic reinforcement layer, respectively. The stone columns are idealized by stiffer springs. The nonlinear behavior of granular fill and soft soil is considered. Consolidation of the soft soil due to inclusion of stone columns has also been included in the model. The results obtained by using the present model when compared with the reported results obtained from laboratory model tests shows very good agreement. The effectiveness of geosynthetic reinforcement to reduce the maximum and differential settlement and transfer the stress from soft soil to stone columns is highlighted. It is observed that the reduction of settlement and stress transfer process are greatly influenced by stiffness and spacing of the stone columns. It has been further observed that for both geosynthetic-reinforced and unreinforced cases, the maximum settlement does not change if the ratio between spacing and diameter of stone columns is greater than 4.     

油罐地基压缩层厚度的研究

变形控制是在软土上建造油罐的一个重要环节，而如何确定压缩层厚度一直是影响沉降计算的一个关键因素。本文列出了根据不同取法所算得的压缩层厚度以及相应的沉降量，并把现场实测值与其进行比较，以此来研究对于油罐地基，怎样确定其压缩层厚度才比较合理。     

Centrifuge simulation of the consolidation characteristics of capped marine sediment beds

Marine sediment capping is a technique where clean sand or sediment is placed over contaminated sediment to reduce the migration of contaminants to the environment. Environmental regulations have limited the use of in situ sediment capping due to concerns about the contaminant migration through the cap. A series of centrifuge tests were conducted to simulate the effects of consolidation settlement of capped marine sediment. This study describes the testing and monitoring of the centrifuge tests. The results from the centrifuge tests are interpreted and compared to predictions made by the PSDDF computer program, which can qualitatively estimate the consolidation settlement of capped marine sediment.

Centrifuge tests were utilized to predict the consolidation of marine sediment caused by the placement of a capping layer. The centrifuge tests used the modeling of models technique to verify that correct modeling procedures were utilized. In this study, the maximum deviation between the centrifuge test results and PSDDF prediction was 20%. Thus, designers should utilize PSDDF consolidation settlement results with caution. Dye tracer studies showed the importance of consolidation-induced advective transport of contaminants. Thus, the capping layer must be appropriately designed to reduce the effects of consolidation-induced advective transport. This may be accomplished by adding a reactive barrier or geosynthetic barrier layer to the cap design.     

离心模型试验模拟塑料排水板处理软土地基的方法和应用

采用大型土工离心机对某工程近海软土地基上堤坝施工期及运行期进行了模拟。试验中采用停机加载法模拟分级施工加载过程, 原型中采用塑料排水板固结法处理软土地基, 模型中则根据固结过程相似的原理, 换算成等效圆截面排水体, 在模型制作中采用等效透水滤芯进行模拟。根据激光位移传感器和孔隙水压力传感器数据可以得出相应原型软土地基的沉降特性和孔隙水压力变化情况。根据试验得到的沉降曲线, 采用"经验双曲线法"推算出了地基最终沉降, 然后得出按沉降推算的分层地基平均固结度随时间的变化。对比试验模拟得到的软土地基固结度和理论计算结果, 二者基本接近, 表明塑料排水板模拟方法用于离心模型试验是可行的。      

Nonlinear Behaviour of an Embankment on Floating Stone Columns

This paper describes a benchmark problem for a floating stone columns supported embankment. In the 3D model, a thick soft subsoil deposit represented by Singapore marine clay was treated by floating stone columns to support a 2.0 m high embankment. The 3D numerical results were compared among different constitutive models such as the hardening soil model, soft soil model, modified Cam-clay model and Mohr-Coulomb model, highlighting the effect of non-linearity in the overall performance of the ground structure. The advanced constitutive soil models allow more realistic soil response to be investigated. From the numerical results, the Mohr-Coulomb model with elastic-perfectly plastic stress-strain response was able to give comparable total settlements but failed to demonstrate some intrinsic behaviours obtained with advanced soil models. These intrinsic behaviours include the mode of failure, displacements profile, stress transfer mechanism and rate of consolidation. This benchmark example has given us an improved understanding of the performance of floating stone columns when a more realistic soil behaviour is modelled.     

Numerical analysis of the installation effects on the behaviour of soft clay improved by stone columns

This paper addresses the installation effects of stone columns in soft soils. Focus is made on the lateral expansion of stone material using the vibro displacement and substitution techniques by means of numerical simulations. The behaviour of reinforced soil after stone column installation is investigated to show how the properties of soft soils can be improved prior to final loading. The effect of such an improvement on the prediction of reinforced soil settlement is evaluated. The axisymmetric unit cell model (UCM) served for the comparison between numerical predictions made by the Mohr-Coulomb and hardening soil constitutive laws adopted for the soft soil. An equivalent group of end bearing columns model was investigated in the axisymmetric condition to predict the settlement of reinforced soil by adopting the Mohr-Coulomb constitutive model for soft clay. The reduction of settlements predicted by the unit cell and group of columns models, due the improvement of the Young’s modulus of soft clay, were compared. It is concluded that a significant reduction of settlement is expected when the group of columns model is considered.     

Settlement evaluation of soft clay reinforced with stone columns using the equivalent secant modulus

A method is proposed to evaluate settlement of soft clay reinforced with stone columns. Finite element analyses were carried out using 15-noded triangular elements with PLAXIS. A drained analysis was carried out using Mohr?CCoulomb??s criterion for soft clay, stones, and sand. The stress due to column installation has been considered in the analysis. At the interface between the stone column and soft clay, interface elements have been used. The settlement ratio (SR) of the soil has been estimated using the equivalent secant modulus. The results are compared with those available in the literature, and the advantages of the numerical analysis were highlighted. Based on the results of this analysis, the SR decrease with compaction surrounding soft soil, but decrease of SR is mainly due to a stiffer column material in soft clay.